Color sensor simulating standard source illuminant

ABSTRACT

Disclosed is an apparatus for measuring on-line the color and color-related properties of a moving sheet. Contrast ratio reflectance measurements are made for providing opacity corrections substantially in real time for a full color spectrum. An optical color sensor in accordance with the invention includes a pair of synchronized spectrometers, the first spectrometer being aligned to view a region of the sheet backed with a highly reflective (&#34;white&#34;) material and the second spectrometer being aligned to view a region of the sheet backed with a highly absorptive (&#34;black&#34;) material. The use of two spectrometers permits substantially simultaneous &#34;black&#34; and &#34;white&#34; measurements for a full color spectrum. The optical color sensing system further includes two light sources, a flashlamp and a continuously energized tungsten filament lamp. Light beams from the two sources are combined to form a sheet-illuminating third beam approximating the D65 standard source. The intensity of the flashlamp is electronically controlled to maintain the balance of UV to visible light that characterizes the standard source. The color sensor further includes a sheet backing system including a rotatable standard wheel carrying a white standard tile. Provision is made to permit rotation of the standard wheel and to standardize the sensor off-sheet while maintaining isolation of the white standard tile from the paper mill environment. The sheet backing system includes a paper guide plate defining an annular vortex space into which air is introduced from a pressurized source. A low pressure region thereby produced in the vortex space draws the paper sheet toward the guide plate. At the same time, circulating air spirals outwardly from the vortex space to form a thin air film or air bearing between the sheet and the paper guide. Sheet flutter is thereby minimized and damage to the sheet is prevented.

FIELD OF THE INVENTION

The present invention relates generally to sensors providing on-line,scanning measurements of such properties as color, whiteness, brightnessand fluorescence of a traveling sheet of material such as paper.

BACKGROUND OF THE INVENTION Opacity Correction

In the quality laboratory of a modern paper mill, color, brightness,whiteness, and fluorescence of the product are conventionally measuredon a multiple sheet "pad" of the paper, rather than on a single sheet.If only a single sheet is measured, the results will be influenced byboth the partial transparency of the sheet and the reflectance of thebacking against which the sheet is observed. Furthermore, the "infinitepad" value is usually what the end customer is concerned with, sincethis is typically how the customer will view the end product. However,these measurement conditions cannot be reproduced in-situ in themanufacturing process, where an "on-line" color sensor can view only asingle thickness of the product.

Two strategies have been employed to improve the agreement of on-linecolor measurements with laboratory "pad" measurements. The firststrategy, an example of which is disclosed in U.S. Pat. No. 4,715,715,is to back the sheet with an opaque material which approximates thecolor and optical scattering power of the paper being manufactured. Ineffect, this strategy reproduces infinite pad conditions. Themeasurement error at each wavelength will be proportional to themismatch between artificial and real "pad" spectral reflectance andinversely proportional to the square of the spectral transmittance ofthe single sheet. This strategy works well for sheets of medium to lowtransparency (<20%), since only modest agreement between backing andproduct is required. For less opaque paper or in the case of frequentlychanging product targets it is often difficult to keep the backing inclose enough agreement with the product to insure good sensorperformance.

The second strategy is to measure the sheet spectral reflectivity twice,once backed with a highly reflective (i.e., "white") material, and oncebacked with a highly absorptive (i.e., "black") material. From theseindependent measurements, the spectral transparency can be determinedand the infinite pad spectral reflectivity calculated according to theKubelka-Munk theory. An example of an apparatus for measuring dark andbright reflectances in succession is disclosed in U.S. Pat. No.4,944,594. The apparatus of that patent includes a sheet backing systemcomprising an optical gating means that absorbs substantially all of thetransmitted portion of the incident radiation when electronicallyswitched to a dark state and reflects substantially all of thetransmitted portion of the radiation when switched to a bright state.This approach does not require maintenance of product related backingsand is therefore to be generally preferred. However, if the twomeasurements occur at significantly different times on significantlydifferent parts of the paper, then the difference between the "blackbacked" and "white backed" spectral reflectivities may be due not onlyto the transparency of the sheet but to product (color and transparency)non-uniformity as well. If such variations are present, the calculationunder the Kubelka-Munk theory will fail. On a modern paper machine, theweb travels at 50 feet per second or more, and the sensor itself may beon a scanner moving 15 inches per second across the traveling sheet. Inaccordance with systems of the prior art in which the backing must beswitched from "black" to "white" between these measurements, themeasurements will thus inevitably be made on very different parts of thesheet. In these systems, the results of numerous reading sequences aretypically combined in an attempt to average out the effect of productcolor non-uniformity. The final measurement update rate is thennecessarily low and does not permit accurate, rapid control of colorduring fabrication of the paper.

Attempts have been made to make simultaneous "black" and "white" backedmeasurements. For example, U.S. Pat. No. 3,936,189 discloses atristimulus colorimeter for on-line monitoring of the color, opacity andbrightness of a moving web such as paper having an area illuminated by alight source. The colorimeter of the '189 patent includes fourphotometers each incorporating a filter to duplicate the ICI tristimulusfunctions for measuring the tristimulus values of the light incident onthese detectors; a brightness detector having a 457 nm filter(brightness, according to the '189 patent, being defined as reflectanceat a source wavelength of 457 nm); and an opacity detector fitted with aY response filter. The '189 patent further includes a sheet backingelement including a quartz "shoe" providing both black and whitebackgrounds. The black background is provided by a cavity under thequartz shoe while the white background is provided by a white stripe onthe surface of the shoe. The photometers are so oriented that theoptical axes of the four color tristimulus value detectors and thebrightness detector are directed toward a portion of the illuminatedarea overlying the black background while the optical axis of theopacity detector is directed towards a portion of the illuminated areaoverlying the white stripe. The output of the opacity detector and theoutput of the tristimulus value detector providing the tristimulus Ymeasurement are used to provide a value of contrast ratio reflectancefor correcting color luminosity or brightness to infinite pad backing.However, because the sensor of the '189 patent employs an opacitycorrection strategy that only corrects for one of the three colorcoordinates which is insufficient to fully define a color, a true colorcorrection can only be estimated. Except for neutral colors, even the Ycorrection is in error since the Y-value is a weighted average over awavelength band and the transparency correction is a non-linear functionof "black" and "white" backed reflectivities which must be applied ateach wavelength before computing the Y-value.

Accordingly, instead of simply a calculation of the luminosity orbrightness of the sheet for infinite backing, what is needed is awavelength-by-wavelength opacity correction before computing all colorcoordinates, that is, a full color correction for opacity at everywavelength. Moreover, such correction should be made in real time topermit immediate on-line adjustments to be made in the papermakingprocess.

Measurement of Fluorescent Properties

U.S. Pat. No. 4,699,510 discloses an on-line color sensor for measuringthe color of a moving sheet of paper which contains fluorescentwhitening agents (FWA). Fluorescent whitening agents typically absorbthe violet and ultraviolet energies of incident light and re-emit theseenergies in the blue range of the visible spectrum to give the paper awhiter appearance. The '510 patent discloses techniques for determiningthe color spectrum of such treated paper if illuminated by a definedsource such as the CIE D65 (North Sky Daylight) standard source. The D65standard source has an energy distribution which, compared to otherstandard sources such as CIE source C, is relatively bright in the300-400 nm range; consequently, paper with fluorescent whitening agentsis likely to appear bluer if illuminated by a D65 source.

The color sensor of the '510 patent has two sources of illumination, onean ultraviolet source which emits light primarily in the excitation bandof fluorescent whitening agents, the other a visible light source withan emission spectrum approximating a CIE standard source which alsoemits a significant amount of light in the UV or excitation range ofFWA. The first source is rapidly switched on and off while the secondsource remains on continuously. Differences in data obtained while thefirst source is on and while it is off is used to compute the FWAefficiency or effective FWA concentration. Further, a corrected colorspectrum can be determined which would be obtained if the sample wereilluminated by a defined or standard source. Analysis of the spectrum oflight reflected and emitted from the sheet is measured by a spectrometerwhich is periodically calibrated off-sheet by means of a plurality ofstandards carried on a stepper motor driven wheel under the sheetpassline. The standards include a white standard moved into position bythe stepper motor to calibrate the sensor. After calibration with thewhite standard, the color sensor reads the surface of a fluorescentstandard sample with known fluorescent properties to determine theexcitation energy in both the UV and incandescent sources and ultimatelyto obtain a color spectrum that is corrected to a true standard definedsource. As the light sources age, however, the balance of UV to visiblelight spectra which characterizes the standard source changes. Insystems of the prior art, compensation for this change is made by a UVblocking filter inserted part way into the flashlamp beam. By adjustingthe physical position of the filter, the balance of the UV to visiblelight can be adjusted. It would be desirable to avoid the use of suchmoving parts.

Protection of Standardizing Tile

Backing systems of the prior art typically include a housing containinga rotatable block or wheel having three or more equiangularly spacedbacking plates which may be selectively rotated by means of a steppermotor to bring backing plates--including plates with black and whitesurfaces to correct for various levels of opacity and for performingperiodic standardization--into position opposite the optical sensingsystem.

Standardization is performed "off sheet" between scans, that is, withthe heads of the scanner moved to one end of their travel. Typically, atile with a white surface having a known reflective response issubstituted for the paper surface and is presented to the incident beamproduced by the optical sensing system. This periodic standardizationprocedure serves to correct for such disturbances and offsets aselectronic circuit drift, aging of the source lamp and the accumulationof dirt or debris on the optical elements. The optical sensor system isthus calibrated during each such standardization procedure based on theknown response of the system to the surface of the standard tile.

Increasing color sensor precision makes it more important to keep thewhite standard tile very clean and sealed off from the paper millenvironment. The rotatable block or wheel backing systems presently inuse do not adequately protect the standard tile from the build up ofdust or debris on the surface of the tile. The accumulation of foreignmatter on the standard tile surface alter the reflectance properties ofthe surface. Such a contaminated standard tile surface results in faultyon-sheet measurements of the color of the paper surface. In presentsystems, frequent cleaning of the standard tile is required and becauseof the inconvenience in gaining access to the tile for this purpose suchcleaning is often not done with adequate frequency.

Sheet Stabilization

Color sensors include an optical sensing system disposed above the pathof the moving sheet and a backing system positioned opposite the opticalsensing system below the path of the sheet. As color sensors become moreprecise, the paper sheet must remain precisely positioned inrelationship to the optical sensing system. In practice this means thatthe paper sheet must run along a backing tile at a very small, constantflying height. In some prior art color sensors, such as that disclosedin the aforementioned U.S. Pat. No. 3,936,189, the sheet rides incontact with the backing system, referred to as a "shoe", whichfunctions to support the moving sheet at a reference position as ittravels past the optical sensing system. Although such a support canprovide a stable vertical position for the sheet within the gap (definedas the distance from the upper and lower heads carrying the sensing andbacking systems) contact between the sheet and the "shoe" can causetearing or marking of the sheet. Although attempts have been made toprovide air cushions for non-contact measurements, these systems havenot provided the necessary sheet stability and positional precisionwithin the gap between the optical sensing head and the backing system.The inability to control sheet flutter reduces the accuracy of colormeasurements as a result of the sheet moving toward and away from theoptical system and backing member.

SUMMARY OF THE INVENTION Opacity Correction

The present invention provides for on-line measurements of the color andcolor-related properties of a moving sheet of material such as paper.According to one aspect of the present invention, there is provided anapparatus for making contrast ratio reflectance measurements from whichopacity corrections may be made substantially in real time for a fullcolor spectrum.

The color sensor of the present invention uses two independent butsynchronized spectrometers, the first spectrometer being aligned to viewa region of the sheet backed with a highly reflective ("white") materialand the second spectrometer being aligned to view a region of the sheetbacked with a highly absorptive ("black") material. Importantly, the useof spectrometers permits opacity corrections to be made for a full colorspectrum. Moreover, because two independent devices are used, the"black" and "white" measurements can be performed at substantially thesame time, eliminating the non-uniformity problem. Since the backingdoes not need to be moved between measurements, the instrument canperform measurements and opacity corrections substantially in real time,the rate being limited only by the performance of the optics andelectronics, thus allowing very high update rates, generally sufficientto detect product non-uniformities such as streaks due to sizingadhesion problems.

Measurement of Fluorescent Properties

In accordance with another aspect of the present invention, there isprovided an optical color sensing system including two sources, aflashlamp switched on and off and a tungsten filament lamp that iscontinuously energized. Light from the flash lamp and tungsten filamentlamp are combined by a beam splitter to illuminate the sheet. The beamsplitter is selected so that the energy distribution of the combinedbeam approximates a standard source, preferably the D65 standard source.The balance of UV to visible light which characterizes such a standardsource will change, however, as the lamps age. Pursuant to the presentinvention, the system computer is used to control the flashlampintensity based on readings made on the standard fluorescent tile duringthe standardization time. Thus, the invention controls the amount ofultraviolet electronically under computer control. The present inventionhas no moving parts and takes advantage of the ease of computer controland the high stability of base spectrum illumination.

Protection of Standardizing Tile

In accordance with another aspect of the invention, during thestandardization procedure, when the sensor is off-sheet, a lower sheetguide forming part of the backing system is lifted by means of a linearactuator against an upper sheet guide on the optical sensor housing soas to seal off the standard wheel. The wheel is free to rotate andexpose the white standard tile to the upper optical sensing systemwithout ever exposing the standard tile to the paper mill environment.Degradation of measurements is thus avoided and frequent cleaning of thestandard tile is made unnecessary.

Sheet Stabilization

In accordance with another aspect of the present invention, the papersheet flying height over a backing tile is so controlled that flutter,i.e., variations in the vertical position of the paper, is minimized.The flying height of the paper sheet is controlled by porting compressedair into a plenum chamber in the form of an annular groove formed in apaper guide comprising a circular plate. This air then flows through aseries of slots cut tangentially into a circle around the backing tile.As a result of the high-speed circular or vortex flow and conservationof momentum, a low pressure area is formed in the annular region. Thislow pressure area draws the sheet down toward the upper surface of theplate along which surface the paper sheet travels. On the other hand,because of centrifugal force, the swirling air flow spirals outwardlyover the surface of the plate thereby providing an air film between thesheet and the surface of the plate which effectively serves as an airbearing and which allows the paper to run very close to the standardalong a stable passline without touching the tile. Sheet flutter isvirtually eliminated. Since there is no contact between the sheet andthe sheet guides, sheet breaks and marking of the sheet are alsoeliminated. The sheet stabilizer design is single sided, is notsensitive to scanner alignment and the airflow can be easily adjusted tocontrol the magnitude of the force urging the sheet towards the guideplate. The air bearing reduces or eliminates wear and contaminatebuild-up on the backing which can be of critical importance forapplications such as a color measurement. Since no "hold-down" rings orbuttons are required on the upper head to keep the sheet in place,problems with sheet pinching or dirt build-up are avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention, below,when read in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic, front elevation view, partly in section, of asensor system in accordance with the present invention;

FIG. 2 is a cross section view of the system of FIG. 1 as seen along theline 2--2;

FIG. 3 is a top, plan view of a sheet backing system in accordance withthe present invention, as seen along the line 3--3 in FIG. 1;

FIG. 4 is a schematic diagram of a spectrometer that may be used inconnection with the present invention;

FIG. 5 is a block diagram of a circuit for controlling synchronizationof the spectrometer;

FIG. 6 is a schematic diagram of a circuit for controlling theenergization of a flashlamp;

FIG. 7 is a front elevation view, in section, of a backing system inaccordance with the present invention in which the backing system isshown in its on-sheet measurement configuration;

FIG. 8 is bottom view of a guide plate forming part of the sheetstabilizer of the invention, as seen along the line 8--8 in FIG. 7;

FIG. 9 is an enlarged front elevation view, in cross section, to showmore clearly certain features of the sheet stabilizer of the presentinvention.

FIG. 10 is an end elevation view, in cross section, of the backingsystem of FIG. 7 showing certain details of a sheet stabilizer inaccordance with another aspect of the invention;

FIG. 11 is a front elevation view, in section, of the backing system ofFIG. 7 in which the backing system is shown in its off-sheet,standardizing configuration; and

FIG. 12 shows the backing system of FIG. 7 during the off-sheetstandardizing procedure in which the backing wheel is in the process ofrotating from one standardizing tile position to another.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a sensor system 10 in accordance with a preferredembodiment of the invention for measuring on-line the color andcolor-related properties of a moving sheet of material 12, such aspaper, during its fabrication. The sheet has a top surface 12a and abottom surface 12b. The color sensor system 10 includes an opticalsensing system 14 disposed above the paper sheet 12 and a backing system16 positioned under the sheet. In a manner well known in the art, theoptical sensing system 14 is mounted in a housing 15 carried by in theupper head 18 (a portion of the outline of which is shown in brokenlines) of a conventional scanner while the backing system 16 is carriedby the lower head 20 (also shown in broken lines) of the scanner. Thescanner heads 18 and 20 are driven in unison back and forth across thewidth of the sheet in a direction transverse to the direction of sheettravel. The housing 15 has a lower planar sheet-guide surface 19 and acentral opening 21.

The optical system 14 has a first light source 22 for periodicallyilluminating the paper sheet 12. The source 22 includes a xenonflashlamp 24 and band pass filter 26 for transmitting energysubstantially only in the ultraviolet excitation band of fluorescentwhitening agents. The flashlamp 24 is turned on and off rapidly, forexample, at 50 Hz, by a power supply 28 controlled by a computer 30coupled to the supply 28 through a digital-to-analog converter (DAC) 32.As will be described in greater detail below, the computer 30 controlsthe charge on a capacitor (which is triggered to discharge at aflashrate of 50 Hz) and hence the amplitude or intensity of the lightproduced by the flashlamp 24. Light transmitted by the filter 26 passesthrough condenser lens 34 to form a collimated light beam 36 along avertical optical beam axis 38. A portion of the beam 36 passes through abeam splitter 40 where it is combined with light from a second lightsource 42. The light source 42, which produces light principally in thevisible wavelength range, comprises a continuously energized tungstenfilament quartz-halide lamp 44 and color correcting filter 46 whichbalances the spectrum of light emitted by the lamp 44 to approximate theCIE standard illuminant "C". Collimating optics 48 adjacent the filter46 forms a beam 50 directed toward the beam splitter 40 along thehorizontal optical beam axis 52 perpendicular to the axis 38. By way ofexample, the optics 48 may take the form of a lens or a parabolic mirrorbehind the lamp 44.

The UV and visible light beams from the sources 22 and 42 transmittedand reflected, respectively, by the beam splitter 40 form a combinedbeam 54 along the vertical axis 38. The combination of filters 26 and46, flashlamp voltage and beam splitter 40 is chosen to provideapproximately the right balance of ultraviolet wavelengths to visiblewavelengths so that the combined beam 54 approximates the energydistribution of the D65 illuminant standard. During the time that thesource 22 is off, the beam 54 will, of course, contain only the lightenergies produced by the continuously energized second source 42 asmodified or redistributed by the beam splitter 40.

The combined beam 54 is reflected by the outer surface of a mirror 56 inthe shape of a right circular cone whose central, vertical axis iscoincident with the beam axis 38 and whose apex subtends an angle of,for example, about 24.5°. The conical mirror directs the rays outwardlyat about 49° relative to the central axis 38. The rays are thenreflected inwardly toward the sheet 12 at an angle of approximately 45°to the plane of the paper 12 by means of a multi-faceted or polygonalmirror 58. It will be evident to those skilled in the art that othergeometries may be employed so long as the final angle is approximately45°. The mirror 58 is in the form of the frustrum of a regular pyramidconverging as shown in FIG. 1 and having a vertical central axiscoincident with the beam axis 38. With reference now also to FIG. 2, inaccordance with a preferred embodiment of the invention, the mirror 58has twenty-four lateral faces 58-1, 58-2, 58-3, and so forth, inclinedinwardly at an angle, α, of about 2°. The light reflected by themultifaceted mirror 58 passes through the housing opening 21 andconverges onto a generally circular area 60 of illumination on the papersurface 12a. This mirror geometry provides uniform light distribution inthe sample region 60 with excellent optical efficiency. The n-sidedpyramidal mirror 58 is an improvement over the standard "axicon" opticsgeometry which uses a cylindrical or conical mirror and avoids thetendency of the axicon optics arrangement to produce a bright spot atthe center of the area of illumination. As shown in FIG. 2, each face58-n of the mirror 58 reflects light from the conical mirror 56 toilluminate a generally trapezoidal area 60-n on the surface 12a of thepaper, the width of which is approximately twice the width of a mirrorfacet and the length of which is proportioned to the diameter ofcollimated beam 54. With correct choice of these parameters, each facetcompletely illuminates the viewing area 60. The overlapping areas 60-1and 60-2 illuminated by the faces 58-1 and 58-2 of the polygon mirroroverlap, as shown in FIG. 2 for the two mentioned mirror faces, so thatthe net effect of the illumination provided by all of the faces of themirror 58 is the circular area 60 having a substantially uniform lightdistribution.

Because paper is typically translucent, a portion of the light incidenton the sheet 12 is transmitted through the sheet and another portion ofthe light is reflected from the sheet surface 12a. The portion of thelight transmitted through the sheet 12 falls upon the top surface 62a ofa tile 62 adjacent the bottom surface 12b of the sheet. The tile 62serves as the backing element during on-sheet measurements by the colorsensor. As shown in FIG. 3, the measure tile 62 has a segmented surfaceone half (64) being substantially reflective ("white") and the otherhalf (66) being substantially absorbing ("black"). The use of asegmented black/white backing together with dual spectrometers (as willbe described) allows simultaneous measurements of sheet reflectivity atmultiple wavelengths. The Kubelka-Munk formulae or other known methodsmay then be used by the computer 30 to compute a transparencycompensation to arrive at the "infinite pad" reflectivity at eachwavelength from the reflectivities at each wavelength measured for thesheet backed with "black" and "white" backings. (The correctionalgorithm does not require "black" or "white" to be perfectly absorbingand reflecting, only that their respective reflective spectra areknown.) Such simultaneous measurement with both "black" and "white"backings ensures that changes in sheet transparency over time does notaffect the determination of infinite pad reflectance. Correction is madeusing the reflectivities measured by the dual spectrometers at eachwavelength, which is superior to using a single "opacity" correctionsince transparency is wavelength dependent.

The optical sensing system 14 also includes a lens 70 positioned abovethe sheet 12 in vertical alignment with the white segment 64 of themeasure backing tile 62. The lens 70 collects the portion of lightreflected by the top sheet surface 12a together with the portion of thelight transmitted through the sheet and reflected by the white part ofthe tile and retransmitted through the sheet. The total reflected light(having a bright reflectance intensity) is focused by the lens 70 on theend of a first fiber optics bundle 72. Similarly, a lens 74 in verticalalignment with the black segment 66 of the measure tile 62 collects thetotal reflected light (having a dark reflectance intensity) emitted bythe portion of the sheet overlying the black segment 66 and focuses iton the end of a second fiber optics bundle 76. The fiber optical bundles72 and 76 transmit light to first and second spectrometers 78 and 80,respectively. With reference now also to FIG. 4 which shows a schematicrepresentation of the spectrometer 78, each of the spectrometers 78 and80 may be a commercially available unit such as the Zeiss MiniatureMonolithic Spectrometer (MMS), an exceedingly compact unit incorporatinga diffraction grating 82 that separates light incident thereon into itscomponent wavelengths and a linear photodetector array 84 which producesan electrical representation of the intensity distribution of thedifferent portions of the spectrum.

Generally, the video or pixel outputs of the detectors of thespectrometers 78 and 80 (also denoted as S1 and S2 in FIG. 1) areclocked by logic on the computer processor board into an ADC and storedsequentially in random access memory. The digitization sequence, whichproceeds at a 200 kHz rate, interleaves the two spectrometer videoreadouts: S1/Pixel1, S2/Pixel1, S1/Pixel2, S2/Pixel2, and so forth. Thisresults in measurement of the spectra from spectrometers 78 and 80simultaneously within 5 μsec.

More specifically, the linear photodetector array 84 (FIG. 4) comprisesa series of diodes each of which generates an analog electrical or pixelsignal the magnitude of which corresponds to the intensity of light inthe range of wavelengths directed to that particular diode. Thephotodetector array of each spectrometer is arranged to provide anoutput in the form of a serial stream of pixel signals. FIG. 5 showsschematically a data acquisition logic circuit 200 for processing andsynchronizing the outputs of the spectrometers 78 and 80 and forproviding a flashlamp trigger signal. The circuit 200 includes a clockand timing circuit 202 having a pair of outputs 204 and 206 providingsynchronizing pixel advance signals to the photodetector arrays of thespectrometers 78 and 80. The pixel advance signals alternately clock outthe pixel output signals from the spectrometers. The pixel signals fromeach spectrometer are integrated by resettable pulse integrators 208 and210 and are alternately applied to a multiplexer 212 controlled by aclock signal from the clock and timing circuit 202. Pixel signalsproduced by the two spectrometers alternately appear at the output ofthe multiplexer and are converted to digital form by ananalog-to-digital converter 214. The digitized pixel signals are storedin a shared memory 218 whose pointer is advanced by the clock and timingcircuit 202 after each digitized pixel signal is stored. The clock andtiming circuit 202 resets each pulse integrator 208, 210 after eachconversion by the ADC 214. The clock and timing circuit 202 interruptsthe microprocessor 216 when the entire spectra for both spectrometershave been converted and stored in memory. The computer 30 then analyzesthe digitized signal arrays and computes the reflectance for eachwavelength for each of the two ("black" and "white") backing conditionsand the true color of the paper. An output 86 (FIG. 1) from the computer30 can be used to control the color of the paper being produced.

As already indicated, the optical components are selected so as toprovide a combined beam 54 from the sources 22 and 42 having adistribution of energy that approximates the D65 standard source. Thisdistribution will change, however, as the lamps 24 and 44 age resultingin a change in the energy distribution manifested by change in theUV-to-visible light balance of the combined beam 54. In accordance withone aspect of the invention, the computer 30 is used to control theintensity of the flash from the flashlamp 24 and therefore the amount ofUV radiation based on readings made on the standard fluorescent tileduring the off-sheet standardization procedure.

Generally, the intensity of the flashlamp flash is determined by thevoltage applied to the flashlamp 24. The flashlamp power supply chargesan energy storage capacitor to a voltage proportional to a controlvoltage, Vref. The computer outputs Vref to the lamp power supply bymeans of a DAC. In this fashion the amount of UV may be tuned tomaintain a UV to visible light balance consistent with the standardsource (D65) wavelength distribution or any other desired illuminantdistribution.

A preferred embodiment of a capacitance discharge circuit 220 forcontrolling the intensity of the flashlamp 24 is shown in FIG. 6. Thiscircuit includes an SCR flashlamp trigger circuit 222 controlled bytrigger signals generated by the clock and timing circuit 202 (FIG. 5).The trigger circuit 222 is connected to the trigger electrode 224 of theflashlamp 24. The circuit 220 includes a high voltage generator 226 forproducing an output signal proportional to the reference voltage Vrefapplied to an input 228 of the high voltage generator 226. By way ofexample, a five volt signal at the input of the high voltage generatorproduces 500 volts at the output terminals of the generator. This outputvoltage charges a capacitor 230 with an energy proportional to thesquare of the output voltage which in turn is proportional to Vref. Atrigger pulse from the flashlamp trigger circuit 222 initiates thebreakdown in the gap of the flashlamp 24 and the stored energy in thecapacitor 230 is discharged in the flashlamp plasma.

The output from the digital-to-analog converter 32 (FIG. 1) supplies thereference voltage, Vref, to the high voltage generator 226. Thisreference voltage may be selected for flashlamp voltage control inseveral ways. For example, at the time the sensor 10 is standardizedoff-sheet, a tile with a known amount of fluorescence is measured. Ifthe measured value is less than the known amount, then the output of DAC32 is increased by the computer 30. If more than the known amount ismeasured then the DAC output is decreased. If necessary, after the firstadjustment the measurement can be repeated until agreement is reached.In practice it is usually possible to predict the correct adjustment ona single iteration. Alternatively, a pair of diodes, one with awavelength selecting filter for the UV and the second with a filter forthe blue, monitor the incident light. The ratio of the UV to blueintensities obtained either electronically or by computer calculationcan be used to control the lamp voltage to hold the UV to blue ratio ata preselected value.

The backing system 16 includes an outer, generally cylindrical casing100 secured to the lower scanner head 20 by means of a square base 101.The cylindrical housing has a vertical central axis 102. When thescanner heads 18 and 20 are properly aligned, the axis 102 of the outercylinder casing 100 is in alignment and coincides with the verticaloptical axis 38. Mounted inside the outer casing 100 is a backing wheel104 rotatable about a horizontal shaft 106 by means of a stepper motor(not shown) coupled to a drive gear 107 mounted on the shaft 106 (FIGS.7 and 10). The backing wheel 104 has identical bores 108 spacedequiangularly about the shaft 106 and each of which receives acylindrical, cup-shaped insert 110 carrying a disk shaped standardizingtile about the open end thereof. These tiles include the above-mentionedblack/white measure tile 62; a white standard tile 112; a verificationtile 114; and a white fluorescent standard tile 116. The backing wheel104 is rotatable by the stepper motor to four discrete positions forbringing the various tiles into the uppermost position adjacent thesheet passline.

The backing system 16 further includes a sheet guide 120 in the form ofa circular plate having an upper horizontal sheet guide surface 122, alower surface 124, a beveled outer edge 126 and a central, circularopening 128. The sheet guide is secured to a cylindrical base plate 130including an inwardly projecting flange 132 (FIG. 9) having an innersurface 133 defining a central opening 134 (FIG. 3) concentric with theopening 128 in the sheet guide 120 but having a smaller diameter thanthat of the opening 128. The diameter of the opening 134 isapproximately the same as the outer diameter of the inserts 110, whichare identical, so as to permit an insert to project upwardly through theopening 134 with minimum clearance between the insert and the innerflange surface 133 to prevent debris from entering the interior of thecasing. In this connection, it will be understood that the outer casingextends about the backing wheel 104, as shown by the broken lines (FIG.7). In this fashion, the standard tiles are protected from the millenvironment during on-sheet measurements.

As best seen in FIG. 8, formed in the lower surface 124 of the sheetguide 120 is a circular groove 136. Also formed in the lower surface 124is a pair of diametrically opposed recesses 138 each opening into thecircular groove 136. A plurality of narrow passages in the form ofbores, or preferably, slits 140 oriented substantially tangent to thecentral opening 128 provide communication between the groove 136 and theopening 128. In the embodiment shown in the drawings, eight (8) slits140 are provided.

As best seen in FIGS. 1, 2, 4, 7 and 10, in the on-sheet measurementconfiguration of the backing system 16, the insert 110 carrying theblack/white measure tile 62 is in its operational position, projectingthrough the flange opening 134 with the upper surface 62a of the tile 62flush or coplanar with the upper guide surface 122 of the sheet guide120. In this on-sheet measurement configuration of the backing system16, an annular space 142 is defined between the outer surface of theinsert 110 and the guide plate central opening 128 (FIG. 9).

With reference to FIG. 10, the base plate 130 includes diametricallyopposed air inlet ports 144 in communication with the recesses 138 inthe sheet guide 120. Each port 144 is connected by a tube 146 to apressurized air supply (not shown).

With the color sensor in its on-sheet, measure configuration, airintroduced under pressure through the ports 144 flows, via the circulargroove 136 and tangential slits 140 into the annular space 142 aroundthe tile support insert 110. The tangential orientation and smallcross-sectional area of the slits 140 impart a high speed circulatoryflow to the air in the annular space 142. As the result of conservationof momentum, a low pressure area is formed in the central portions ofthe vortex, that is, in the region immediately about the backing tileinsert 110. This low pressure area causes the portion of the paper sheetoverlying the tile to be urged down toward the outer tile surface 62a.At the same time, however, the centrifugal forces acting on thecirculating air in the annular space 142 causes air to escape and spiraloutwardly along the upper sheet guide surface 122. The result is anequilibrium in the position of the paper, with a thin air film or airbearing being present in a gap 148 (FIG. 9) between the bottom surface12b of the paper sheet on the one hand and the sheet guide and standardtile surface 62a on the other. Thus, the paper surface 12b runs veryclose to the measure tile surface 62a without contacting it along asheet passline that is highly stable, i.e., subject to minimum flutter.The thickness of the air film can be readily controlled by adjusting theair flow into the ports 144 by means of a needle valve or the like (notshown). The sheet stabilizer of the invention is single sided and thereduced pressure around the tile insert 110 prevents any dirt or debrisfrom entering the housing 100 via any small gap that may exist betweenthe insert 110 and the inner surface 133 of the flange 132.

Provision is also made to facilitate rotation of the backing wheel 104and to standardize the sensor off-sheet while maintaining isolation ofthe white standard tile from the paper mill environment. In thisconnection, the cylindrical base plate 130 is slidably received within abore 100a formed in the casing 100. The base plate 100 thus forms apiston vertically displaceable within the bore 100a. The base plate 130is mounted on a pair of connecting rods 150 slidably received invertical bores 152 formed in the casing 100 (FIG. 11). The rods 150 aresecured at their lower ends to a cross member 154 which in turn isconnected to the piston of a pneumatic cylinder 156. Accordingly, up anddown displacement of the piston will cause a corresponding verticaldisplacement of the base plate 130 and sheet guide 120. During on-sheetmeasurements, the piston in the cylinder 156, and hence the sheet guide120, are in their retracted positions as shown, for example, in FIGS. 1,7, 9 and 10 with, as already stated, the top surface 62a of the measuretile 62 being coplanar with the sheet guide upper surface 122.

In preparation for standardization, the scanner heads are movedoff-sheet, that is, to an extreme position of their travel beyond one orthe other side edge of the sheet 12. With reference now to FIG. 11, oncethe scanner heads are in the standardization position, compressed air isported to the pneumatic cylinder 156 thereby raising the sheet guide 120against the lower guide surface 19 surrounding the opening 21 in theoptical sensor housing 15 thereby effectively sealing off the backingwheel 104. With the measure tile insert 110 now free of the constraintof the base plate opening 134, the backing wheel 104 is free to berotated, as shown in FIG. 12, to expose the surface of the whitestandard tile 112 to the optical sensing system 14 without exposing thewhite tile to the paper mill environment. In this fashion, the whitetile 112 is at all times--during on-sheet measurement as well as duringoff-sheet standardization--isolated from the ambient contaminatingenvironment of the paper mill.

Various standard tiles can be rotated into reflectance measuringposition during standardizing. When the standardizing procedure iscomplete, the backing wheel 104 is rotated by the stepping motor tobring the measure tile 62 to the top position. The pneumatic cylinder156 is actuated to retract the sheet guide 120 to its on-sheet position(for example, FIGS. 1 and 7) so that scanning of the paper sheet canonce again take place.

What is claimed is:
 1. A color sensor for measuring the fluorescentproperties of a sample containing a fluorescent whitening agent, thesensor comprising:a first light source, the light emitted by the firstlight source being primarily in the ultraviolet excitation band of saidfluorescent whitening agent; a second light source, the light emitted bythe second light source being primarily in the visible portion of thespectrum; means for combining the light emitted by the first and secondlight sources to provide a light beam for illuminating a portion of thesample, the combined light beam having a spectral distributionapproximating a predetermined illuminant; at least one detector forreceiving light from the illuminated portion of the sample and providingan output indicative of the intensity of the received light as afunction of wavelength; a power supply connected to the first lightsource for energizing the first light source and switching the firstlight source between on and off states; and an electrical circuitconnected to the power supply of the first light source and responsiveto the output of said at least one detector, for controlling the powersupply to vary the intensity of the light emitted by the first lightsource to correct for deviations of the spectral distribution of thesample-illuminating beam from that of the predetermined illuminant.
 2. Acolor sensor, as defined in claim 1, in which:the electrical circuitcorrects for deviations in the ultraviolet-to-visible light balance ofthe sample-illuminating light beam from that of the predeterminedilluminant.
 3. A color sensor, as defined in claim 1, in which:the powersupply applies an energizing voltage to the first light source andwherein the electrical circuit controls the amplitude of the energizingvoltage applied to the first light source.
 4. A color sensor, as definedin claim 1, in which:the second light source includes a continuouslyenergized incandescent lamp and color correcting filter, the lightemitted by the second light source approximating the CIE standardilluminant "C"; and wherein the predetermined illuminant approximatesthe D65 illuminant standard.
 5. A color sensor, as defined in claim 1,in which:the light combining means includes a beam splitter positionedto receive the light emitted by the first and second light sources, thebeam splitter transmitting the light emitted from one of the lightsources and reflecting the light emitted from the other of the lightsources, the transmitted and reflected light providing thesample-illuminating beam having a spectral distribution approximatingsaid predetermined illuminant.
 6. A method for measuring the fluorescentproperties of a sample containing a fluorescent whitening agent,comprising the steps of:producing a first light beam primarily in theultraviolet excitation band of the fluorescent whitening agent;producing a second light beam primarily in the visible portion of thespectrum; combining the first and second light beams to produce a thirdlight beam for illuminating a portion of the sample, the third lightbeam having a spectral distribution approximating a predeterminedilluminant; detecting the light emerging from the illuminating portionof the sample and providing a signal representing the intensity of thedetected light as a function of wavelength; switching the first lightbeam between on and off states; and electronically controlling theintensity of the first light beam in response to said signal to vary theintensity of the first light beam to correct for deviations of thespectral distribution of the third light beam from that of thepredetermined illuminant.
 7. A method, as defined in claim 6, inwhich:the controlling step corrects for deviations in theultraviolet-to-visible light balance of the third light beam from thatof the predetermined illuminant.
 8. A method, as defined in claim 6, inwhich:the first light beam is produced by an energizing voltage; and inwhich: the intensity of the first light beam is controlled by varyingsaid energizing voltage.
 9. A method, as defined in claim 6, inwhich:the first light beam has a spectral distribution approximatingthat of the CIE standard illuminant "C"; and the predeterminedilluminant approximates the D65 illuminant standard.